This invention relates to a cement additive containing a superplasticizer and bentonite and more particularly to a cement additive for eliminating tackiness of a cement composition and preventing the cement composition from cracking at an initial stage upon drying before the cement composition is hardened.
As compared with common plasticizers, such as ligninsulfonate or polyol type plasticizers, now in popular use in ready-mixed concrete plants, superplasticizers have desirable properties in that a cement composition or concrete having only low set retarding and air entraining properties, a high water reducing ratio and high strength may be easily produced even when a larger amount of the superplasticizer is added to the cement. However, the concrete admixed with the superplasticizer exhibits properties totally different in the unhardened state from those of the concrete admixed with the common plasticizers. Specifically, the former exhibits strong tackiness and non-bleeding. Thus, the following problems are presented in construction works employing such concrete.
More specifically, on account of strong tackiness, the concrete sags or slides down along a tilt if it exists on the construction site, so that the construction works are obstructed. In addition, on account of non-bleeding properties, the water in the concrete is lost on drying before the concrete is hardened, thus causing cracking upon drying due to plastic shrinkage. While such cracking occurs after lapse of more or less time interval since the time of casting, depending on the ambient temperature, humidity and the wind force prevailing at the time of construction, it may occur after lapse of a shorter time interval, such as 20 to 30 minutes, in an extreme case, with the width of crack being as large as 2 to 5 mm. In addition, because of strong tackiness and non-bleeding properties, coarse aggregates float on the concrete surface, while a skin is formed on drying on the concrete surface so that the troweling operation is obstructed, thus resulting in only poor surface finishing. Even when water is sprayed on the concrete surface to improve its surface finishing, the aggregates cannot be sunk because floating of the aggregates in the concrete is mainly caused by tackiness of the concrete. On the contrary, the superplasticizers in the concrete may float to cause an irregular color and adversely affect the surface finish such as uniform color and smoothness.
It is thus seen from the above that although the superplasticizers are superior to ordinary plasticizers in certain aspects when used for preparation of high strength concrete, they have not come into customary use in civil engineering and construction works in general, only because of the above problems. Although it is strongly desired to solve these problems, the art or technology which provides direct solution of these problems has not been evolved.
On the other hand, strength of the concrete admixed only with the superplasticizer ceases to increase when the concrete is dried during curing, with the resulting strength being fluctuated appreciably and occasionally reaching only a lower value depending on the degree of drying. Thus, the strength cannot be designed in large-sized civil engineering and construction works where it is not possible to perform sufficient wet curing.
It is known to add calcium sulfates, above all, Type II anhydrous calcium sulfate, in order that a sufficient design strength be obtained even upon drying, while it is also known to produce a high strength concrete by using citric acid or salts thereof, carbonates of alkali metals or siliceous materials in combination, as shown for example Japanese Laid-open Patent Publication No. 167460/1983. However, the above problems resulting from strong tackiness of the concrete employing the superplasticizer cannot be solved by these methods.
Although the art of improving concrete strength by using the superplasticizer and calcium sulfates simultaneously is now in popular use in factories producing concrete articles with the use of steam curing, the fact is that only a centrifugal casting mold is actually employed which is not affected by various problems resulting from concrete tackiness, while the art cannot be applied extensively to pre-cast members for which finishing is required, because the finishing properties are low while cracking on drying may be produced during pre-curing.